Making a Laser in Cycles - Straight Beam of Light?
First Solution:

I have a line-laser and a plane-laser node setup.  

For the line-laser:




For the plane-laser:




Second Solution:

Now that you have asked and I tried to remember,
I got a solution that is easier to understend.  

You can get the plane-laser when using
dot-product instead cross-product.
Then higher strength is required.  





Important Edit

Since I answered this, Blender was updated several times, and I had to update my solution also. An important note is that this solution only works with area lights, since the normal is only provided for those. Other light sources are not capable of providing their direction to the node system. I will contact the devs about this, since the normal has no meaning for other lights now, it could contain the direction of the object.

Some artifacts appear near the edge of the area light when using constant strength, therefor a slight adjusment is required. This can also be used with a point light, when the bottom most node is used instead of the geometry normal. For a point light you can actually skip the subtract.



Here is everything nicely organized and nodes grouped: